1. Field of the Invention
This invention relates to a process for preparing orthosilicic acid tetraalkyl esters having 2 to 6 carbon atoms in the ester group. More especially, this invention relates to a process for preparing such orthosilicic tetraalkyl esters which process is characterized by exceptionally improved volume-time yields. This invention is particularly directed to a process for preparing such an orthosilicic acid tetraalkyl ester wherein the process is carried out in the presence of a surface active substance or a compound containing a methoxy group or is carried out under pressure at a temperature above the boiling point of the reaction mixture. The invention is concerned especially with methods for obtaining orthosilicic acid alkyl esters in higher volume-time yields than has been provided by former methods.
2. Discussion of the Prior Art
It is known to prepare orthosilicic acid tetraethyl esters in accordance with West German Pat. No. 1,768,781 by reacting metallic silicon with ethanol in the presence of high-percentage alkali ethylate solutions. This method has the disadvantage that relatively low volume-time yields are achieved by the application thereof.
The above-mentioned disadvantages are offset to a certain extent by the method of West German Pat. No. 1,793,222. In this method the reaction is performed in the presence of the desired orthosilicic acid tetralkyl ester. This brings about a lower concentration of the alkali alcoholates in the reaction mixture.
Both methods are very well suited for the preparation of silicic acid tetramethyl ester. In the preparation of silicic acid tetraethyl ester and of the higher esters, however, difficulties are encountered which result in a reduction of the material yield and of the volume-time yield. The causes of these difficulties remain unexplained to this date.
However, in the preparation of silicic acid tetraethyl ester and the higher esters, with the silicon placed first in the reactor, the speed of the reaction diminishes constantly in the course of the reaction, and the components do not react completely, for this reduction of speed is more rapid than the reduction of the concentration of the components. In the preparation of silicic acid tetraethyl ester, for example, the result is that only about 40% of the silicon charged enters the reaction, and then, despite an excess of ethanol, the reaction comes to a halt.
While not wishing to be bound by any theory, it is believed by some that the decrease of the speed of reaction during the process is explainable in that products form during the process which adversely affect the surface of the metallic silicon. This adverse affect on the surface of the silicon, it is believed, is eliminated pursuant to the invention.
It is an object of this invention, therefore, to provide a process which speeds the course of the reaction of metallic silicon with a C.sub.2 -C.sub.6 alcohol en route to the preparation of orthosilicic acid tetraalkyl esters having 2 to 6 carbon atoms.